This invention relates to a magnetic recording/reproducing apparatus, and more particularly to an apparatus of the kind described above which comprises a field memory so as to improve various special playback functions of the apparatus.
A magnetic recording/reproducing apparatus comprising three magnetic heads and one field memory for realizing various special playback modes including a still picture playback mode, a slow motion playback mode and a fast forwarding playback mode is disclosed in the specification and drawings of Japanese Patent Application No. 61-186825 (1986), on the basis of which U.S. application Ser. No. 047217 was filed May 8, 1987 (Korean patent application was filed on May 7, 1987). By the incorporation of the field memory in the disclosed magnetic recording/reproducing apparatus, pictures of various special playback modes can be satisfactorily reproduced with greatly reduced noise in spite of the fact that only three magnetic heads are mounted on a rotary cylinder. For example, reproduction according to the still picture playback mode can be attained by writing a video signal of a desired field in the field memory and continuously reading out the video signal from the memory. Also, reproduction according to the slow motion playback mode can be attained by sequentially writing a video signal of noise-free fields in the field memory and continuously reading out the written information from the field memory until the magnetic heads are moved to scan succeeding noise-free fields. Further, in the case of reproduction according to the fast forwarding playback mode, the magnetic heads forming a double-azimuth head pair for one channel operate to write a continuous video signll in the field memory, and, after change-over of the channel to the other, the continuous video signal written by the double azimuth head pair is read out from the field memory.
Therefore, a picture having a satisfactory quality free from noise and fluctuation can be reproduced according to such a special playback mode. However, the prior art magnetic recording/reproducing apparatus has not been still fully satisfactory in that the fact that the period of an address reference signal changes at the transient time of mode change-over is no taken into consideration, and out-of-vertical synchronization tends to occur when an address reset signal is merely changed over in response to the mode change-over. The prior art problem will be described with reference to FIG. 6. FIG. 6 shows the relation between the phase of such reference signal RT and that of a vertical synchronizing signal V during the field-memory writing and reading operations. The reference signal RT is used as a basic signal for controlling a servo system driving a cylinder motor or a capstan motor, and the vertical synchronizing signal V is added to a video signal read out from a magnetic tape or the field memory. These two signals RT and V are synchronized by the function of a servo block and are locked to maintain a phase difference therebetween of about 6.5H (horizontal period) as shown. That is, in the writing operation, the address signal appears at time earlier by 6.5H than a pulse of the vertical synchronizing signal V, and a digital video signal corresponding to one field between that pulse and the next pulse of the vertical synchronizing signal V is written in the field memory. On the other hand, in the reading operation, the same phase relationship as that in the writing operation is maintained between the signals RT and V when the playback mode is maintained at one of the special playback modes. However, when the playback mode is changed over to another during the reading operation, the phase relationship between the two signals RT and V changes from that established in the writing operation. Herein, the change-over of the playback mode to another during the reading operation refers to the case where a video signal is held in a transient state. That is, the memory is placed in its readable state immediately before the playback mode is changed over to another, and the playback mode is then changed over. Since the period of the reference signal changes depending on the selected playback mode, the phase relationship between the two signals RT and V in the reading operation differs from that used in the writing operation. Various cases are considered to occur as a result of suhh a mode change-over. For example, the problem of out-of-vertical synchronization on the displayed picture occurs when the speed of the magnetic tape is changed from a high speed to a low speed. (Herein, it is defined that the tape speed is high when the magnetic tape travels in the positive direction.) An example of the above case is the change-over from the normal playback mode to the reverse review (search) mode. In the reverse review mode in which the tape speed is three times as high as that in the normal playback mode, the period of the reference signal RT is extended from 262.5H to 268.5H. That is, the video signal stored in the field memory is read out at the period of 268.5H in the reading operation. However, the signal corresponding to 262.5H only is written in the field memory, and an undefined data is generated from the field memory in the remaining period of 6H. Depending on the value of the undefined data, a synchronizing signal separating circuit in a television receiver may mistake the data as the vertical synchronizing signal. In such a case, out-of-vertical synchronization resulting in vertical jitter of the displayed picture occurs.